Rollup merge of #78208 - liketechnik:issue-69399, r=oli-obk

replace `#[allow_internal_unstable]` with `#[rustc_allow_const_fn_unstable]` for `const fn`s

`#[allow_internal_unstable]` is currently used to side-step feature gate and stability checks.
While it was originally only meant to be used only on macros, its use was expanded to `const fn`s.

This pr adds stricter checks for the usage of `#[allow_internal_unstable]` (only on macros) and introduces the `#[rustc_allow_const_fn_unstable]` attribute for usage on `const fn`s.

This pr does not change any of the functionality associated with the use of `#[allow_internal_unstable]` on macros or the usage of `#[rustc_allow_const_fn_unstable]` (instead of `#[allow_internal_unstable]`) on `const fn`s (see #69399 (comment)).

Note: The check for `#[rustc_allow_const_fn_unstable]` currently only validates that the attribute is used on a function, because I don't know how I would check if the function is a `const fn` at the place of the check. I therefore openend this as a 'draft pull request'.

Closes #69399

r? @oli-obk

Author: JohnTitor

compiler/rustc_attr/src/builtin.rs

@@ -1013,13 +1013,28 @@ pub fn allow_internal_unstable<'a>(
     sess: &'a Session,
     attrs: &'a [Attribute],
 ) -> Option<impl Iterator<Item = Symbol> + 'a> {
-    let attrs = sess.filter_by_name(attrs, sym::allow_internal_unstable);
+    allow_unstable(sess, attrs, sym::allow_internal_unstable)
+}
+
+pub fn rustc_allow_const_fn_unstable<'a>(
+    sess: &'a Session,
+    attrs: &'a [Attribute],
+) -> Option<impl Iterator<Item = Symbol> + 'a> {
+    allow_unstable(sess, attrs, sym::rustc_allow_const_fn_unstable)
+}
+
+fn allow_unstable<'a>(
+    sess: &'a Session,
+    attrs: &'a [Attribute],
+    symbol: Symbol,
+) -> Option<impl Iterator<Item = Symbol> + 'a> {
+    let attrs = sess.filter_by_name(attrs, symbol);
     let list = attrs
         .filter_map(move |attr| {
             attr.meta_item_list().or_else(|| {
                 sess.diagnostic().span_err(
                     attr.span,
-                    "`allow_internal_unstable` expects a list of feature names",
+                    &format!("`{}` expects a list of feature names", symbol.to_ident_string()),
                 );
                 None
             })
@@ -1029,8 +1044,10 @@ pub fn allow_internal_unstable<'a>(
     Some(list.into_iter().filter_map(move |it| {
         let name = it.ident().map(|ident| ident.name);
         if name.is_none() {
-            sess.diagnostic()
-                .span_err(it.span(), "`allow_internal_unstable` expects feature names");
+            sess.diagnostic().span_err(
+                it.span(),
+                &format!("`{}` expects feature names", symbol.to_ident_string()),
+            );
         }
         name
     }))

compiler/rustc_feature/src/active.rs

@@ -210,6 +210,11 @@ declare_features! (
     /// it is not on path for eventual stabilization).
     (active, no_niche, "1.42.0", None, None),
 
+    /// Allows using `#[rustc_allow_const_fn_unstable]`.
+    /// This is an attribute on `const fn` for the same
+    /// purpose as `#[allow_internal_unstable]`.
+    (active, rustc_allow_const_fn_unstable, "1.49.0", Some(69399), None),
+
     // no-tracking-issue-end
 
     // -------------------------------------------------------------------------

compiler/rustc_feature/src/builtin_attrs.rs

@@ -379,6 +379,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         allow_internal_unstable, AssumedUsed, template!(Word, List: "feat1, feat2, ..."),
         "allow_internal_unstable side-steps feature gating and stability checks",
     ),
+    gated!(
+        rustc_allow_const_fn_unstable, AssumedUsed, template!(Word, List: "feat1, feat2, ..."),
+        "rustc_allow_const_fn_unstable side-steps feature gating and stability checks"
+    ),
     gated!(
         allow_internal_unsafe, Normal, template!(Word),
         "allow_internal_unsafe side-steps the unsafe_code lint",

compiler/rustc_mir/src/transform/check_consts/mod.rs

@@ -79,9 +79,13 @@ pub fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
         || Some(def_id) == tcx.lang_items().begin_panic_fn()
 }
 
-pub fn allow_internal_unstable(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
+pub fn rustc_allow_const_fn_unstable(
+    tcx: TyCtxt<'tcx>,
+    def_id: DefId,
+    feature_gate: Symbol,
+) -> bool {
     let attrs = tcx.get_attrs(def_id);
-    attr::allow_internal_unstable(&tcx.sess, attrs)
+    attr::rustc_allow_const_fn_unstable(&tcx.sess, attrs)
         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 }
 

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -292,7 +292,11 @@ impl Validator<'mir, 'tcx> {
 
             Status::Unstable(gate) if self.tcx.features().enabled(gate) => {
                 let unstable_in_stable = self.ccx.is_const_stable_const_fn()
-                    && !super::allow_internal_unstable(self.tcx, self.def_id().to_def_id(), gate);
+                    && !super::rustc_allow_const_fn_unstable(
+                        self.tcx,
+                        self.def_id().to_def_id(),
+                        gate,
+                    );
                 if unstable_in_stable {
                     emit_unstable_in_stable_error(self.ccx, span, gate);
                 }
@@ -807,7 +811,7 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                     }
 
                     // Calling an unstable function *always* requires that the corresponding gate
-                    // be enabled, even if the function has `#[allow_internal_unstable(the_gate)]`.
+                    // be enabled, even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
                     if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == gate) {
                         self.check_op(ops::FnCallUnstable(callee, Some(gate)));
                         return;
@@ -821,7 +825,7 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
 
                     // Otherwise, we are something const-stable calling a const-unstable fn.
 
-                    if super::allow_internal_unstable(tcx, caller, gate) {
+                    if super::rustc_allow_const_fn_unstable(tcx, caller, gate) {
                         return;
                     }
 
@@ -967,8 +971,8 @@ fn emit_unstable_in_stable_error(ccx: &ConstCx<'_, '_>, span: Span, gate: Symbol
         )
         .span_suggestion(
             attr_span,
-            "otherwise `#[allow_internal_unstable]` can be used to bypass stability checks",
-            format!("#[allow_internal_unstable({})]\n", gate),
+            "otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks",
+            format!("#[rustc_allow_const_fn_unstable({})]\n", gate),
             Applicability::MaybeIncorrect,
         )
         .emit();

compiler/rustc_passes/src/check_attr.rs

@@ -85,6 +85,10 @@ impl CheckAttrVisitor<'tcx> {
                 self.check_export_name(&attr, span, target)
             } else if self.tcx.sess.check_name(attr, sym::rustc_args_required_const) {
                 self.check_rustc_args_required_const(&attr, span, target, item)
+            } else if self.tcx.sess.check_name(attr, sym::allow_internal_unstable) {
+                self.check_allow_internal_unstable(&attr, span, target, &attrs)
+            } else if self.tcx.sess.check_name(attr, sym::rustc_allow_const_fn_unstable) {
+                self.check_rustc_allow_const_fn_unstable(hir_id, &attr, span, target)
             } else {
                 // lint-only checks
                 if self.tcx.sess.check_name(attr, sym::cold) {
@@ -719,6 +723,55 @@ impl CheckAttrVisitor<'tcx> {
             }
         }
     }
+
+    /// Outputs an error for `#[allow_internal_unstable]` which can only be applied to macros.
+    /// (Allows proc_macro functions)
+    fn check_allow_internal_unstable(
+        &self,
+        attr: &Attribute,
+        span: &Span,
+        target: Target,
+        attrs: &[Attribute],
+    ) -> bool {
+        debug!("Checking target: {:?}", target);
+        if target == Target::Fn {
+            for attr in attrs {
+                if self.tcx.sess.is_proc_macro_attr(attr) {
+                    debug!("Is proc macro attr");
+                    return true;
+                }
+            }
+            debug!("Is not proc macro attr");
+        }
+        self.tcx
+            .sess
+            .struct_span_err(attr.span, "attribute should be applied to a macro")
+            .span_label(*span, "not a macro")
+            .emit();
+        false
+    }
+
+    /// Outputs an error for `#[allow_internal_unstable]` which can only be applied to macros.
+    /// (Allows proc_macro functions)
+    fn check_rustc_allow_const_fn_unstable(
+        &self,
+        hir_id: HirId,
+        attr: &Attribute,
+        span: &Span,
+        target: Target,
+    ) -> bool {
+        if let Target::Fn | Target::Method(_) = target {
+            if self.tcx.is_const_fn_raw(self.tcx.hir().local_def_id(hir_id)) {
+                return true;
+            }
+        }
+        self.tcx
+            .sess
+            .struct_span_err(attr.span, "attribute should be applied to `const fn`")
+            .span_label(*span, "not a `const fn`")
+            .emit();
+        false
+    }
 }
 
 impl Visitor<'tcx> for CheckAttrVisitor<'tcx> {

compiler/rustc_passes/src/check_const.rs

@@ -87,7 +87,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
 
         let is_feature_allowed = |feature_gate| {
             // All features require that the corresponding gate be enabled,
-            // even if the function has `#[allow_internal_unstable(the_gate)]`.
+            // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
             if !tcx.features().enabled(feature_gate) {
                 return false;
             }
@@ -105,8 +105,8 @@ impl<'tcx> CheckConstVisitor<'tcx> {
             }
 
             // However, we cannot allow stable `const fn`s to use unstable features without an explicit
-            // opt-in via `allow_internal_unstable`.
-            attr::allow_internal_unstable(&tcx.sess, &tcx.get_attrs(def_id))
+            // opt-in via `rustc_allow_const_fn_unstable`.
+            attr::rustc_allow_const_fn_unstable(&tcx.sess, &tcx.get_attrs(def_id))
                 .map_or(false, |mut features| features.any(|name| name == feature_gate))
         };
 

compiler/rustc_span/src/symbol.rs

@@ -894,6 +894,7 @@ symbols! {
         rustc,
         rustc_allocator,
         rustc_allocator_nounwind,
+        rustc_allow_const_fn_unstable,
         rustc_args_required_const,
         rustc_attrs,
         rustc_builtin_macro,

library/alloc/src/lib.rs

@@ -72,6 +72,7 @@
 #![allow(explicit_outlives_requirements)]
 #![allow(incomplete_features)]
 #![deny(unsafe_op_in_unsafe_fn)]
+#![cfg_attr(not(bootstrap), feature(rustc_allow_const_fn_unstable))]
 #![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(test, feature(test))]
 #![cfg_attr(test, feature(new_uninit))]

library/alloc/src/raw_vec.rs

@@ -150,7 +150,8 @@ impl<T> RawVec<T, Global> {
 impl<T, A: AllocRef> RawVec<T, A> {
     /// Like `new`, but parameterized over the choice of allocator for
     /// the returned `RawVec`.
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn new_in(alloc: A) -> Self {
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr: Unique::dangling(), cap: 0, alloc }

library/core/src/lib.rs

@@ -63,6 +63,7 @@
 #![warn(missing_debug_implementations)]
 #![allow(explicit_outlives_requirements)]
 #![allow(incomplete_features)]
+#![cfg_attr(not(bootstrap), feature(rustc_allow_const_fn_unstable))]
 #![feature(allow_internal_unstable)]
 #![feature(arbitrary_self_types)]
 #![feature(asm)]

library/core/src/num/int_macros.rs

@@ -2045,7 +2045,8 @@ assert_eq!(
             #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
             // SAFETY: const sound because integers are plain old datatypes so we can always
             // transmute them to arrays of bytes
-            #[allow_internal_unstable(const_fn_transmute)]
+            #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_transmute))]
+            #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_transmute))]
             #[inline]
             pub const fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
                 // SAFETY: integers are plain old datatypes so we can always transmute them to
@@ -2193,7 +2194,8 @@ fn read_ne_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
             #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
             // SAFETY: const sound because integers are plain old datatypes so we can always
             // transmute to them
-            #[allow_internal_unstable(const_fn_transmute)]
+            #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_transmute))]
+            #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_transmute))]
             #[inline]
             pub const fn from_ne_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
                 // SAFETY: integers are plain old datatypes so we can always transmute to them

library/core/src/num/uint_macros.rs

@@ -1803,7 +1803,8 @@ assert_eq!(
             #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
             // SAFETY: const sound because integers are plain old datatypes so we can always
             // transmute them to arrays of bytes
-            #[allow_internal_unstable(const_fn_transmute)]
+            #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_transmute))]
+            #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_transmute))]
             #[inline]
             pub const fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
                 // SAFETY: integers are plain old datatypes so we can always transmute them to
@@ -1951,7 +1952,8 @@ fn read_ne_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
             #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
             // SAFETY: const sound because integers are plain old datatypes so we can always
             // transmute to them
-            #[allow_internal_unstable(const_fn_transmute)]
+            #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_transmute))]
+            #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_transmute))]
             #[inline]
             pub const fn from_ne_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
                 // SAFETY: integers are plain old datatypes so we can always transmute to them

library/core/src/slice/mod.rs

@@ -88,7 +88,8 @@ impl<T> [T] {
     #[rustc_const_stable(feature = "const_slice_len", since = "1.32.0")]
     #[inline]
     // SAFETY: const sound because we transmute out the length field as a usize (which it must be)
-    #[allow_internal_unstable(const_fn_union)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_union))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_union))]
     pub const fn len(&self) -> usize {
         // SAFETY: this is safe because `&[T]` and `FatPtr<T>` have the same layout.
         // Only `std` can make this guarantee.

library/core/src/str/converts.rs

@@ -157,7 +157,8 @@ pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_str_from_utf8_unchecked", issue = "75196")]
-#[allow_internal_unstable(const_fn_transmute)]
+#[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_transmute))]
+#[cfg_attr(bootstrap, allow_internal_unstable(const_fn_transmute))]
 pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
     // SAFETY: the caller must guarantee that the bytes `v` are valid UTF-8.
     // Also relies on `&str` and `&[u8]` having the same layout.

library/core/src/str/mod.rs

@@ -219,7 +219,8 @@ impl str {
     #[rustc_const_stable(feature = "str_as_bytes", since = "1.32.0")]
     #[inline(always)]
     #[allow(unused_attributes)]
-    #[allow_internal_unstable(const_fn_transmute)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_transmute))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_transmute))]
     pub const fn as_bytes(&self) -> &[u8] {
         // SAFETY: const sound because we transmute two types with the same layout
         unsafe { mem::transmute(self) }

library/core/src/task/wake.rs

@@ -130,7 +130,8 @@ impl RawWakerVTable {
     #[rustc_promotable]
     #[stable(feature = "futures_api", since = "1.36.0")]
     #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
-    #[allow_internal_unstable(const_fn_fn_ptr_basics)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn_fn_ptr_basics))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn_fn_ptr_basics))]
     pub const fn new(
         clone: unsafe fn(*const ()) -> RawWaker,
         wake: unsafe fn(*const ()),

library/proc_macro/src/bridge/client.rs

@@ -401,7 +401,8 @@ fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
 }
 
 impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn expand1(f: fn(crate::TokenStream) -> crate::TokenStream) -> Self {
         extern "C" fn run(
             bridge: Bridge<'_>,
@@ -414,7 +415,8 @@ impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
 }
 
 impl Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream> {
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn expand2(
         f: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
     ) -> Self {
@@ -459,7 +461,8 @@ impl ProcMacro {
         }
     }
 
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn custom_derive(
         trait_name: &'static str,
         attributes: &'static [&'static str],
@@ -468,15 +471,17 @@ impl ProcMacro {
         ProcMacro::CustomDerive { trait_name, attributes, client: Client::expand1(expand) }
     }
 
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn attr(
         name: &'static str,
         expand: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
     ) -> Self {
         ProcMacro::Attr { name, client: Client::expand2(expand) }
     }
 
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn bang(
         name: &'static str,
         expand: fn(crate::TokenStream) -> crate::TokenStream,

library/proc_macro/src/bridge/scoped_cell.rs

@@ -35,7 +35,8 @@ impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
 pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
 
 impl<T: LambdaL> ScopedCell<T> {
-    #[allow_internal_unstable(const_fn)]
+    #[cfg_attr(not(bootstrap), rustc_allow_const_fn_unstable(const_fn))]
+    #[cfg_attr(bootstrap, allow_internal_unstable(const_fn))]
     pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
         ScopedCell(Cell::new(value))
     }

library/proc_macro/src/lib.rs

@@ -18,6 +18,7 @@
     test(no_crate_inject, attr(deny(warnings))),
     test(attr(allow(dead_code, deprecated, unused_variables, unused_mut)))
 )]
+#![cfg_attr(not(bootstrap), feature(rustc_allow_const_fn_unstable))]
 #![feature(nll)]
 #![feature(staged_api)]
 #![feature(const_fn)]